It is often necessary in integrated circuit (IC) design to generate frequency synthesized signals. For example, a copy of a master clock may be divided to provide timing and control signals for internal IC functions that operate at different frequencies. The master clock copy typically is divided by a conventional divide-by-n circuit having a plurality of flip-flops. The divide-by-n circuit provides internal IC functions with a clock having a frequency equal to 1/n the frequency of the master clock.
A synchronization problem, however, may arise when switching between the master clock copy and a divided master clock copy. The propagation delay added to the divided master clock copy by, for example, the flip-flops in the divide-by-n circuit, may spoil the synchronization between the master clock copy and the divided master clock copy. Moreover, if a multiplexer is used to select between the master clock copy and the divided master clock copy, additional propagation delay could be added to these clocks which may exacerbate the synchronization problem.
The prior art addresses the synchronization problem by providing a second clock source that operates at twice the frequency of the master clock to derive synchronized clocks. For example, some prior art may generate a synchronized, divided clock by dividing the second clock source by two and four, respectively. Unfortunately, the need to select between a divided and a non-divided copy of a master clock while maintaining synchronization between such clocks is not addressed by such prior art. Moreover, for applications that push the limits of semiconductor technology, it may be impossible to provide a second clock source that operates at twice the frequency of the master clock. Other prior art uses a phase-locked loop (PLL) to synchronize frequency synthesized clocks. Using a PLL, however, may prove too costly for certain applications.
Accordingly, there is a need for multiplexed synchronization circuits for switching between frequency synthesized signals without losing synchronization between such signals.